Internal sample attenuator counting (ISAC). A new technique for separating and measuring bound and free activity in radioimmunoassays.

I describe a new method for the separation and counting of bound and free activity in radioimmunoassays. Particles containing a radiation-absorbing (attenuating) material are added to the assay. They shield the radiation from either the antibody-bound or the free radioligand. This obviates such manipulations conventionally involved in the separation and counting steps of radioimmunoassays as centrifugation and decanting. Bismuth oxide is used as the attenuator. Particles with different properties are described. In one type, bismuth oxide is combined with active charcoal in an agarose matrix and serves as an adsorbant for the free radioligand. In another type bismuth oxide is trapped within a polyacrylamide matrix to which antibodies are coupled. This particle can be used with a first- or a second-antibody bound activity. Application of the technique is illustrated with radioimmunoassays for thyroxin, triiodothyronine, human choriogonadotropin, and lutropin (luteinizing hormone).